How to signal "not implemented yet"?
You may use the todonotes-gem
There is a documentation with some examples.
It doesn't implement an exception, but a logging mechanism and a possibility for temporary solutions.
Looks like the original answer, which suggested raising NotImplementedError, was removed. I'll take a crack at it: write documentation.
Don't add code that's just a placeholder. You wouldn't want folks coding against that API so don't even give them a chance (yourself included). Instead, document the road map you are currently planning on in the class and/or README. Then be open to it changing. Odds are by the time you get around to solving whatever problem is in the road map you'll have new thoughts on what is the appropriate solution. I think this is the right course of action in any language/framework, but I think Ruby in particular encourages us to not write code that you don't plan on having executed.